This invention relates to wind-driven generators.
The extraction of energy from the wind is an ancient art. Perhaps the first attempts at harnessing the power of wind were the primitive horizontal windmills of 10th century Persia. Throughout the following centuries may refinements and improvements have been made in this field to obtain a more efficient conversion of wind force to usable mechanical energy.
Following the development of electrical generators many efforts have been made to use wind power to drive electrical generators and thus convert wind force directly electrical energy. A complete wind-driven generator system consists of a rotor system which will be rotatably driven by the wind, an electrical generator, a mechanical power transmission coupling the rotor system to the generator and an electrical system for storing electric energy when generated and/or distributing such energy to point of use.
Several fundamental principles govern the design of wind-driven generators. The actual power available from the wind is proportional to the cube of the wind speed. If the wind speed is doubled, eight times as much power is available. It is also theoretically impossible in an open-air windplant to recover more than 59.26% of the kinetic energy of the wind. Thus, if the rotor itself is 75% efficient and the generator is 75% efficient, then no more than 33.34% of the wind energy can be converted to electricity. This amount of energy will be further decreased by the efficiency of the mechanical transmission between the rotor and generator. A further important factor is that the amount of energy that can be captured is dependent on the disk area swept by the rotors.
Considerable work has been done in the air foil design of wind-driven rotors to increase the efficiency thereof. A wind-driven rotor is characterized by its "tip-speed ratio," which is the ratio of the peripheral speed of the blade tips to the wind speed at maximum power. For example, a rotor having a tip-speed ratio of six will have a peripheral speed of 150 miles per hour inn a 25-miles-per-hour wind when full power is being generated. The tip speed usually doubles when the load is removed from the rotor, and in the case above, the peripheral tip speed would increase to 300 miles per hour. A typical low-tip speed ratio rotor is the familiar multi-blade wind pump, which has a tip-speed ratio of about one. These machines are not suitable for generating electricity due to their low efficiency and slow speed, but perform very well in the operation of piston pumps, due to their high starting torque and lack of dependence on sophisticated feathering devices.
A considerable challenge to the designer of a wind-driven generator system is the choice of tip-speed ratio of the rotor system with relation to the operating speed of the electric generator. Conventional generators or alternators are readily available but are designed for rotational speed of 1200-1800 rpm. Special low-speed generators are available but are considerably more expensive.
For a given diameter rotor, the rotational speed of the rotor will be proportional to its tip-speed ratio at any wind speed below that at which a speed governor comes into operation. If a low-speed ratio is used, then the speed of rotation will be low. In order for the generator to be driven at the desired high speed either a complex and costly gearbox or a costly low-speed generator will be required. On the other hand, if a high tip-speed ratio is selected to reduce the step-up gearing requirements then costly mechanisms will be required to feather the rotor blades in high winds to keep the rotor from exploding.
The greatest rate of movement of a rotor is, of course, present in the outer extremity of the rotor. In an effort to reduce the gearing requirements, attempts have been made to put a rim around the rotor and extract the power through belts, chains or gears. However, none of these appear to have been successful, and none are commercially available. The reason for this is that the aerodynamic and frictional losses associated with belt or chain drives are prohibitive, and there is no economic advantage to forming gear teeth on the rotor periphery.